SEARCHING FOR A SOLUTION TO E-BIKE FIRES
The lithium-ion battery fire issue in New York City has garnered a lot of attention, and rightfully so. Unfortunately, New York City’s misfortune is spreading to other municipalities, both in the United States and internationally.
This crisis highlights many aspects, some good and some bad, of the emerging category of micromobility within the worldwide transportation infrastructure. Micromobility, e-bikes in particular, can and will help combat many critical issues that we as a global community are working hard to address, climate change being the most crucial.
But efficient and reliable e-bikes require the carriage of energy-dense power supplies. Lithium-ion batteries are lightweight, compact, and incredibly energy-dense. Being lightweight and compact are critically important attributes for use with a powered two-wheeler. Properly designed, manufactured, used, and charged lithium-ion battery packs are ideal for this application. Lithium-ion batteries enable the storage of sufficient energy, and the delivery of sufficient power, to make e-bikes practical and useful for any number of personal transportation missions.
It's no surprise that lithium-ion batteries are the energy source of choice, at least for the foreseeable future, for these important and useful vehicles.
However, poorly-designed battery packs, using sub-standard battery cells, assembled without due care, utilizing poorly-designed battery management systems, charged by incompatible battery chargers, and abused in service, are susceptible to catastrophic failure, primarily due to thermal runaway and subsequent fire.
Poorly-designed and manufactured battery packs subjected to extremely harsh service conditions are the root cause of the New York fire crisis.
Lithium-ion battery fires are incredibly dangerous. They generate temperatures exceeding 2500°F. They generate copious quantities of extremely toxic gases including hydrogen fluoride, hydrogen cyanide, and carbon monoxide. An e-bike battery pack is comprised of numerous individual battery cells that can become dislodged from the battery pack and jet around, being propelled by the gas venting from the cell like small rocket engines, igniting secondary fires as far away as 10 to 15 meters (33 to 50 feet) from the failing battery pack.
Lithium-ion battery fires are almost impossible to extinguish. Water will not extinguish a battery pack fire. It can only serve to slow propagation. Once a battery fire appears to have been extinguished, residual energy remaining in affected cells can reignite, causing the original fire to perpetuate.
On balance, an e-bike battery fire not only presents a significant life-threatening danger to users and bystanders, but also to the first responders and firefighters charged with dealing with them.
Therefore, the development and implementation of short- and long-term solutions to this issue is of paramount importance.
A long-term solution is already in process. As stated above, quality battery packs are significantly less likely to fail catastrophically. If a quality battery pack is properly charged and used in service, the chances of a dangerous failure are almost zero.
Our primary task then becomes ensuring that only quality battery packs are developed and distributed for use here and abroad.
Safety standards for lithium-ion batteries have been developed and promulgated by competent agencies, most notably, Underwriters Laboratories (UL). When a pack is designed and manufactured such that it complies with the requirements of UL standard UL 2271, that battery pack can be relied upon (when properly used and maintained) to deliver safe and reliable service for the duration of its useful life.
However, UL 2271 is a voluntary standard. But the crisis in NYC, having drawn the attention of local, state, and federal regulators, has stimulated the promulgation of current and pending laws requiring that all micromobility battery packs be tested and certified compliant (by Nationally Recognized Testing Laboratory) with UL 2271 before they can legally be sold as original equipment and/or replacement battery packs for e-bikes.
These laws, when enforced, will ensure that this crisis will ultimately end. But until such time that the hundreds of thousands of substandard, poor-quality battery packs currently in circulation are purged from use in this country and elsewhere, the problem will persist.
Efforts to eliminate dangerous packs are under discussion. However the proposals to date are expensive, face difficult logistical hurdles, and will take significant time to implement.
What can we do in the meantime?
It has been suggested that since ending catastrophic battery failures is a long-term goal, our short-term efforts should be directed toward mitigating the effects of the battery fires that we know will occur in the interim.
The most promising strategy (from my perspective) is the development and installation of safe battery pack charging and storage cabinets, and the implementation of battery charging stations using safe storage cabinets and/or other means of containing a battery fire when it occurs. While feasible from a technological point-of-view, these too will be expensive and relatively slow to implement.
It has been suggested that widespread distribution of “battery bags” to users (primarily delivery people, the biggest group of sub-standard battery pack users in NYC) represents a potential short-term solution.
A battery bag is a bag or pouch fabricated from high-temperature and flame-resistant fabric. The proposed use would be to store and charge e-bike batteries in the battery bag, so if a fire occurs, the battery bag could contain the flames and heat generated by a battery fire. While unable to contain the toxic gas generated by a battery pack, it is thought that the benefit to the at-risk personnel in the vicinity would be to buy time to safely evacuate the area and to give first responders time to intervene. Further, the battery bag would hinder fire propagation to the structure adjacent to the failing battery pack.
However, a recently published report titled “Evaluation of Fire Spread and Suppression Techniques in Micro-Mobility Battery Packs” calls that mitigation strategy into question.
The testing described in this report was conducted in conjunction with a very reputable engineering R&D laboratory, Exponent. I have worked with Exponent in the past and can attest to their excellent engineering and scientific expertise and testing capabilities.
The testing, as the report title indicates, evaluated several fire suppression techniques, including the use of a fire blanket. The test results indicated that the use of a fire blanket exacerbates the severity of the battery fire. The blanket apparently hinders heat transfer, resulting in a hotter fire that accelerates cell-to-cell propagation, resulting in a significantly more dangerous situation. Further, the blankets tested were not able to withstand the significant temperatures generated by the battery fire, and the fire actually breached the blankets.
This test did not evaluate “battery charging bags.” But one could extrapolate the results of the blanket test to battery bags given that the use of a bag would present a similar scenario with respect to trapping the heat generated by a burning battery pack, accelerating cell-to-cell propagation, thus worsening the fire.
The good news, from the e-bike industry perspective, is that the testing validated the efficacy of a battery fire mitigation strategy that our industry has been investigating for some months. The tests discovered that by insulating individual 18650 cells in the pack from one another, using a resin introduced into the pack during manufacture (or other insulating media) precluded cell-to-cell propagation. In that manner, if a single cell were to fail, the fire would not propagate to other cells in the pack, resulting in a relatively benign failure.
I anticipate that this mitigation strategy will be adopted by the industry after further evaluation. Unfortunately, this effective solution does not represent a short-term solution and does not help the current New York situation.
I must advise that this is just one test. These conclusions are subject to further evaluation and peer review. Further, not much detail was given regarding the actual construction of the blankets evaluated. Perhaps there are blankets available from other sources that would have done a better job resisting burn-through. Further, this test, as stated, did not evaluate battery bags. But the “trapped heat accelerating propagation” mechanism would still be present.
The propensity of the blanket to trap heat and accelerate propagation will be consistent with the battery bag fire suppression technique. Assuming that a battery bag would exacerbate the situation is logical and sound engineering speculation.
The test report referenced above can be found at:
https://iopscience.iop.org/article/10.1149/2754-2734/ad1a72/pdf
There are no formal safety standards nor recognized testing protocols that evaluate the efficacy of battery charging bags. UL has recently formed UL Technical Committee UL 1487, charged with formulating safety standards for “battery containment enclosures.” I plan to petition this technical subcommittee to include battery bags in their deliberations.
In the meantime, I strongly urge anyone considering the use of a battery storage bag reconsider that plan until such time as formal testing is performed to determine their effectiveness in mitigating the severity of failing e-bike battery packs.
Contact Mike Fritz, mike@humanpoweredsolutions.com.